Gran Turismo 6 would definitely be a big seller, specially since incredible graphics can be pulled off from driving sims (nice way to show off the hardware).
Really, between gt5 p and gt5 they have sold a total of over 14 mill, it would be a smart move to release a gt as a launch game, probs not gonna happen though.
However if my force feedback wheel was to be compatible with gt6 then that would also be very handy.